An Improved Noise Model for SiGe HBT With an Inductive Breakdown Network in the Avalanche Region

In this paper, an improved radio-frequency (RF) noise model is extended for silicon germanium (SiGe) heterojunction bipolar transistors (HBTs) by applying RF avalanche delay mechanism to investigate noise performance in the breakdown region where reliability is a concern. The impact of inductive feedback due to base-collector (BC) junction avalanche breakdown on noise performance is analyzed and explained by utilizing this improved noise model incorporating an inductive network. The avalanche effects on noise sources are verified and explained physically by the dead-space theory. The contributions of electrons and holes to the avalanche noise are taken into account by including an electron- and hole-breakdown network in the presented noise model. The presented method is compared with the conventional approach. When the avalanche noise is absorbed by base and collector noise sources as in the conventional RF noise model without a breakdown network, simulated noise parameters deviate from experimental data at breakdown. Instead, good agreement between measured and simulated noise parameters and S-parameters from the presented RF noise model is achieved in the inductive breakdown and active regions. The presented noise model and the obtained parameters can be applied to SiGe HBT circuits for reliability investigation.